1. Field of the Invention
The present invention relates to a buffer layer included in a photoelectric conversion device and a manufacturing method thereof, a reaction solution for use in manufacturing the buffer layer included in a photoelectric conversion device, a photoelectric conversion device having the buffer layer, and a solar cell using the photoelectric conversion device.
2. Description of the Related Art
Photoelectric conversion devices having a photoelectric conversion layer and electrodes conducting to the photoelectric conversion layer are used in various applications, including solar cells and the like. Most of the conventional solar cells are Si-based cells using bulk single-crystalline Si, polycrystalline Si, or thin film amorphous Si. Recently, however, research and development of compound semiconductor-based solar cells that do not depend on Si has been carried out. Two types of compound semiconductor-based solar cells are known, one of which is a bulk system, such as GaAs system and the like, and the other of which is a thin film system, such as CIS system formed of a group Ib element, a group IIIb element, and a group VIb element, CIGS, or the like. The CI(G)S system is a compound semiconductor represented by a general formula, Cu1-ZIn1-xGaxSe2-ySy (where, 0≦x≦1, 0≦y≦2, 0≦z≦1), and when x=0 the formula represents a CIS system, while when x>0 it represents a CIGS system. The CIS and CIGS are herein collectively denoted as “CI(G)S”.
Conventional thin film photoelectric conversion devices, such as CI(G)S systems and the like, generally include a CdS buffer layer between a photoelectric conversion layer and a translucent conductive layer (transparent electrode) formed thereon. In such a system, the buffer layer is normally formed by CBD (chemical bath deposition) method
Roles of the buffer layer may include (1) prevention of recombination of photogenerated carriers (2) band discontinuity alignment (3) lattice matching (4) coverage of surface unevenness of the photoelectric conversion layer, and the like. For CI(G)S systems and the like, the CBD method, which is a liquid phase method, is preferably used in order to satisfy, in particular, the condition of (4) above, since the photoelectric conversion layer has relatively large surface unevenness.
In view of the environmental burden, Cd-free buffer layers are under study, and as a major component of Cd-free buffer layers, use of zinc systems, such as ZnO systems, ZnS systems, and the like is also under study.
Japanese Unexamined Patent Publication No. 2000-332280 (Patent Document 1) discloses a method of producing a Zn(O, OH, S) buffer layer using a reaction solution that includes a zinc-containing compound, a sulfur-containing compound, and an ammonium salt (claim 1). Patent Document 1 also describes that a reaction solution that includes 0.5 mol/l or less of ammonia is preferably used (claim 2). Patent Document 1 further describes that a reaction temperature of 10 to 100° C. and a pH of 9.0 to 11.0 are preferable (claims 6 and 7).
Japanese Unexamined Patent Publication No. 2001-196611 (Patent Document 2) discloses a method of producing a Zn(S, O) buffer layer using a reaction solution containing zinc acetate, thiourea, and ammonia (Example 3). In Example 3 of Patent Document 2, concentrations of the zinc acetate, thiourea, and ammonia are 0.025M, 0.375M, and 2.5M, respectively.
Japanese Unexamined Patent Publication No. 2002-343987 (Patent Document 3) discloses a method of producing a Zn(S, O, OH) buffer layer using a reaction solution which is a mixture of a solution provided by dissolving zinc salt in ammonia water or ammonium hydroxide water and an aqueous solution provided by dissolving sulfur-containing salt in purified water (claim 1). Patent Document 3 describes that the film forming is performed with a transparency level of the reaction solution of 100% to 50% (claim 1). Patent Document 3 further describes that a reaction temperature of 80 to 90° C. and a pH of 10.0 to 13.0 are preferable (claims 5 and 6).
Japanese Unexamined Patent Publication No. 2003-124487 (Patent Document 4) discloses a method of producing a Zn(S, O) buffer layer by a roll-to-roll process using a reaction solution containing zinc acetate, thiourea, and ammonia. In Example 2 of Patent Document 4, concentrations of the zinc acetate, thiourea, and ammonia are 0.025M, 0.375M, and 2.5M, respectively.
Japanese Unexamined Patent Publication No. 2002-118068 (Patent Document 5) discloses a method of producing a ZnS buffer layer using a reaction solution containing zinc sulfate, ammonia, and thiourea (claim 4). U.S. Pat. No. 7,704,863 (Patent Document 6) discloses a method of producing a buffer layer which includes the steps of dissolving a 0.05 to 0.5 mol/l of zinc sulfate and a 0.2 to 1.5 mol/l of thiourea in distilled water at a temperature of 70 to 90° C., adding about 25% ammonia in the amount of ⅓ of the water, and after the solution becomes transparent, dipping the substrate in the solution for about 10 minutes to maintain the temperature substantially at constant within the time (claim 1).
A literature by D. Johnston et al., “Chemical bath deposition of zinc sulphide thin films using sodium citrate as a complementary complexing agent”, Journal of Materials Science Letters, Vol. 20, pp. 921-923, 2001 (Non-patent Document 1) describes a method of producing a ZnS thin film using a reaction solution containing zinc sulfate, thiourea, ammonia, and sodium citrate. In Non-patent Document 1, the film forming is performed at a reaction temperature of 60 to 80° C.
A literature by H. J. Lee and S. I. Lee, “Deposition and optical properties of nanocrystalline ZnS thin films by a chemical method”, Current Applied Physics, Vol. 7, Issue 2, pp. 193-197, 2007 (Non-patent Document 2) describes a method of producing a ZnS thin film using a reaction solution containing zinc sulfate and thioacetamide. In Non-patent Document 2, the film forming is performed at a reaction temperature of 95° C. with a reaction time of 90 to 120 min.
When providing a buffer layer by CBD method, it is necessary to form a film that well covers an underlayer. Further, in view of the production efficiency and cost, a high reaction speed is preferable in the film forming process for the buffer layer by CBD method.
Japanese Unexamined Patent Publication No. 2007-242646 (Patent Document 7) discloses a method of forming a buffer layer by providing a nucleation site or a growing site, which is a particle of the same kind as or a different kind from that of the buffer layer, and forming the buffer layer with the nucleation site or the growing site as the starting point or the catalyst (claim 1). Further, ZnS is specifically cited as a major component of the particle serving as the nucleation site or the growing site and the buffer layer (claim 8).
As described in Patent Document 7, by growing a CBD film after forming a fine particle layer that functions as the nucleation site or the growing site of crystal growth, catalyst, or the like, the reaction speed of the CBD film forming process may be increased and a film well covering an underlayer may be formed stably by controlling the crystal growth through CBD reaction.
In the CBD reaction, colloidal particles are generated as the reaction progresses and the reaction solution sometimes becomes opaque white. When the white opaque becomes significant, colloidal particles may adhere to the substrate and a uniform film becomes unable to be formed. In such a case, it is necessary to take measures, such as replacing the reaction solution with respect to each or several substrates. For example, Patent Document 5 discloses a method for performing CBD film forming by replacing the reaction solution (claim 1).
Patent Document 3 describes that a Zn system buffer layer is formed within a transparency range of the reaction solution from 100 to 50% (claim 1). Patent Document 3 also describes a method in which transparency of the reaction solution is monitored and a substrate is pulled out of the reaction solution when the transparency becomes 70 to 50% (claims 3 and 4, and the like). Patent Document 3 does not describe the relationship between prescription and transparency, and it is not clear that what prescription may prevent white opaque. The method described in Patent Document 3 can not form a buffer layer having a sufficient thickness if a reaction solution likely to become opaque white is used.
Patent Document 4 discloses a manufacturing apparatus having a means for removing sediment from a reaction solution (claim 9). Removal of sediment may prevent colloidal particles from adhering to the substrate, but the problem of replacement frequency for the reaction solution is not solved since the composition of the reaction solution varies with time from the initial composition.
In order to stably obtain uniform films with less replacement frequencies for the reaction solution, it is preferable that the generation of colloidal particles is prevented as long as possible and the white opaque of the reaction solution is prevented.
The present invention has been developed in view of the circumstances described above, and it is an object of the present invention to provide a reaction solution in which the generation of colloidal particles is prevented as long as possible and the reaction solution is prevented from becoming opaque white, thereby allowing a Zn system buffer layer well covering an underlayer to be formed. It is a further object of the invention to provide a manufacturing method of a Zn system buffer layer well covering an underlayer by preventing colloidal particle adhesion.